1. Field of the Art
The present invention relates to a tri-color separating optical system incorporated in various color image pickup apparatus of high resolution.
2. Prior Art
Video signals representative of an optical image of a subject can be obtained through a color image pickup apparatus, and have various advantages such as the signal processing (e.g. editing, trimming, etc.) and the signal recording/reproducing processing being facilitated by use of an erasable recording medium. In the color image pickup apparatus for generating video signals, an optical image of a subject obtained through an image pickup lens system is separated into three primary colors through a color separating optical system and then focused onto a photoelectric transducer composed of image pickup elements in order to generate electrical image information signals corresponding to the optical image of a subject. The generated image information signals are output in time sequence as video signals. As is well known, various image tubes or various solid state image sensing elements are used as the image pickup elements incorporated into the color image pickup apparatus.
On the other hand, a multi-layer color film is used to record color optical images of a subject.
In passing, it must be mentioned that, recently, various novel television systems (i.e. EDTV/extended frequency range TV, HDTV/high definition TV) have been proposed along with the increasing demand for higher quality and higher resolution reproduced picture. To improve the quality of the reproduced picture, a color image pickup apparatus which can generate video signals of higher picture quality and higher resolution are now required. In the case of color image pickup apparatus using an image tube as the image pickup element however, it is difficult to generate video signals which can reproduce picture images of higher quality and higher resolution. This is because there exists a limit to minimizing the diameter of the electron beam in the image tube. Furthermore, since the target capacitance of the image tube increases with increasing target area, it is impossible to improve the resolution by enlarging the target area. Furthermore, in case of taking moving pictures, the frequency band of video signals is demanded to range from several tens to several hundreds of megahertz (MHz), thus the S/N ratio deteriorates as the resolution of the moving pictures increases.
This can be explained in more detail by saying that two means of generating video signals which can reproduce high-quality and high-resolution pictures by a color image pickup apparatus using an image tube as an image pickup elements, are by decreasing the electron beam diameter of the image tube or by enlarging its target area. However, it is impossible to reduce the electron beam diameter below a certain limit determined by the performance of an electron gun and the structure of the focusing system. Furthermore, when the target area is enlarged by using a large diameter image lens in order to improve the resolution, since the high frequency signal components of output signals of the image tube have a reduced voltage level due to an increase in the garget capacitance, the S/N ratio of the image tube output signals deteriorates markedly. In summary, in the prior art color image pickup apparatus using an image tube, it has been impossible to generate video signals which can reproduce high-quality and high-resolution pictures.
Furthermore, in the case of color image pickup apparatus using solid state image pickup elements, although (pixels) it is required to increase the number of picture elements in order to reproduce high quality and high resolution pictures, there exist other problems in that the frequency of the clock for driving the picture elements increases to as high as 500 to 600 MHz in the case of moving picture cameras and also that the electrostatic capacitance of the driver circuits increases with an increasing number of picture elements. At present, since the upper limit of the frequency of the clock for driving solid state image pickup elements is about 20 MHz, it is therefore impossible to construct a practical apparatus.
To overcome the above-mentioned problems, that is, to generate video signals which can reproduce high-quality and high-resolution pictures, the same applicant has already filed a Patent Application (U.S. Ser. No. 139,005 now U.S. Pat. No. 4,831,452) entitled "Color image pickup apparatus", in which one of the proposed systems comprises means for separating an optical image of a subject obtained through an image lens into two or more optical images different in at least the wavelength range; means for forming each separated optical image at respective light-to-light converting element; and means for reprojecting the optical image information corresponding to optical images of a subject formed on said single light-to-light converting element, onto erasable recording medium, i.e. rewritable recording medium, in the form of optical image information signals on said erasable recording medium in time sequence. In the above-mentioned color image pickup apparatus, since video signals that can reproduce high-resolution pictures can easily be obtained from such reversible namely rewritable recording medium on which optical image information is recorded, it is possible to solve the aforementioned problems involved in the prior-art apparatus. However, there exists a problem in that the construction of this apparatus is rather complicated since each of two or more optical images with different wavelength ranges obtained by separating an optical image of a subject obtained through an image pickup lens, has to be formed on respective different light-to-light converting elements.
In more detail, with reference to FIG. 1, the prior art tri-color separating optical system comprises a subject O, an image pickup lens L, dichroic mirrors D.sub.r, D.sub.b, two total reflection mirrors M.sub.r1 and M.sub.b1, a red-component optical image-forming plane I.sub.r1, a blue-component optical image-forming plane I.sub.b1, and a green-component optical image-forming plane I.sub.g. In the prior art tri-color separating optical system shown in FIG. 1 since the three image-forming planes I.sub.r1, I.sub.b1 and I.sub.g of three optical images obtained through color separation are spatially separated from each other, it has been necessary to use a special light-to-light converting element for forming a plurality of optical images. As shown in FIG. 1, the spatial offsets of these three image-forming planes I.sub.r1, I.sub.b1 and I.sub.g include an axial offset a i.e. a displacement along the light pathes to the image-forming planes, and a transverse offset having a same amount a caused in a direction perpendicular to the light pathes, both offsets are refered to the green-component optical image-forming plane I.sub.g. The axial offset is caused by the transverse offset.
There are other proposed systems disclosed in the same Patent Application (U.S. Ser. No. 139,005), in which separated three(3) color images are focused on a single plane, but these systems require either complicated optical arrangement or expensive optical path-length adjusting members, otherwise an orientation of the focused color images would be opposite each other, i.e. one image would be upright but other two images might be upside-down.
Accordingly, when two or more optical images with different wavelength ranges obtained by separating an optical image of a subject obtained through an image pickup lens are recorded on an erasable recording medium, since the image forming planes of optical images obtained through color separation are either spatially separated from each other, or the orientations of optical images of a subject formed or the image-forming planes are different from each other, it is necessary to use a plurality of different erasable recording media to record plural optical images, thus complicating the apparatus structure.
The above-mentioned problem is common to a case where two or more optical images with different wavelength ranges obtained by separating an optical image of a subject obtained through an image pickup lens are recorded on an irreversible (not erasable but writable once) recording medium such as a monochrome film which records each of the separated color images in the effort of preventing discoloration inherent to a multilayer color film.